New areas of medical study and new clinical applications involving the use of 500 KHz-300 MHz ultrasound imaging are constantly being developed. Ultrasound images made at the high end of this frequency range will have spatial resolutions that approach 20 microns. Initial clinical applications of high frequency ultrasound include imaging the eye, the vasculature, the skin, and cartilage. Such imaging may be used, for example, to determine the vertical growth phase of skin cancers, to distinguish between cancerous tissue and fat in the breast, and to determine quantitative information about the structure of atherosclerotic plaque in arteries.
Future improvements in ultrasound image quality will require the fabrication of ultrasonic transducer arrays using designs and fabrication techniques not heretofore available. More particularly, transducer arrays manufactured with current transducer fabrication technology have limited spatial resolution, restricted scan slice thickness, inadequate phase correction capability, and primitive beam steering for volumetric scanning. To overcome these limitations, the next generation of ultrasonic transducer arrays will need to be multidimensional and operate over a broad range of frequencies.
Two-dimensional (N.times.M) ultrasound imaging arrays are the subject of much research and development due to their potential for overcoming some of the above-described limitations of known one-dimensional (N.times.1) linear arrays. Unfortunately, rapid development and commercialization of 2-D ultrasound imaging arrays has been hampered by difficulties in fabricating the transducer elements with small dimensions and low electrical impedance.
Current ultrasonic transducers are typically fabricated by machining crystals of PZT into the required shape. Then, appropriate matching and backing layers are added, the PZT material is diced, and the electrical connections are made. Dimensions of the individual transducer elements are such that the height is typically one-half the wavelength (the ratio of the speed of sound in the material and the operating frequency), the width is typically one-half the height, the length is approximately fifteen times the width, and the spacing between adjacent elements is approximately one-half the wavelength in the tissue. Since all of the dimensions are in proportion to the wavelength and the wavelength is inversely proportional to the operating frequency, the dimensions are inversely proportional to the operating frequency. As operating frequencies of ultrasonic transducers increase, the dimensions of the individual elements decrease, and the ability to machine the PZT crystal to the correct dimensions becomes difficult. In addition, two-dimensional arrays will require the length and width of each element be comparable. Furthermore, in order to realize increased sensitivity associated with multi-layer structures, the height will consist of perhaps 10 to 20 layers (reducing the dimensions further) which needs to be interspersed with electrodes. Limitations in current transducer fabrication techniques prevent the manufacture of ultrasonic transducer arrays having the sensitivity and absolute and relative dimensions desired.
In an attempt to overcome these limitations in known ultrasonic transducer fabrication techniques, Smith developed an improved multilayer ultrasonic transducer array and techniques for manufacturing such array, as described in U.S. Pat. Nos. 5,329,496 and 5,548,564. These patents describe the subject ultrasonic transducer array as capable of generating operating frequencies in the range 1MHz to 10MHz, and above. However, it is believed operating frequencies in excess of 5MHz have not been achieved with the ultrasonic transducer array described in U.S. Pat. Nos. 5,329,496 and 5,548,564 due to limitations in the materials and processes described in these patents for fabricating the arrays.
More specifically, U.S. Patents Nos. 5,329,496 and 5,548,564 describe techniques for fabricating multilayer PZT transducers using green tape. Fabrication methods based on green tape, and also based on the use of screen printing, have reached the limit of ceramic layer thinness that can be made due to the grain size of the ceramic and due to the poor dimensional tolerances. In addition, the piezoelectric films made from green tapes and screen printing suffer from low material densities due to the fact that they contain binders which need to be removed in a sintering process and which results in significant material shrinkage and internal voids. Fabrication methods based on thin film sol-gel materials are limited due to achievable thickness. These materials are applied in a series of very thin (less than 2 micron thick) layers which are stacked on top of each other with an annealing step between each layer deposition. Even slight thermal mismatches between the piezoelectric layers and the substrate can result in thermal-induced cracking for even moderate thickness films (i.e., greater than 10 microns).
As the center frequency of the transducer array increases, the size of discrete transducer elements decreases and the electrical impedance of each element increases. In order to reduce the impedance, multilayer arrays are preferred. However, it becomes increasingly difficult to provide the necessary electrical interconnections in multilayer arrays. Known manufacturing techniques for multilayer two-dimensional transducer arrays using green tape, screen printing or sol-gel films to form the piezoelectric layers are not believed to permit formation of the complex interconnections necessary in future generation high frequency multilayer ultrasonic transducer arrays.
Attempts have been made to deposit PZT material by sputtering, as reported by K. Screenivas et al. in the article Bulk and Surface Acoustic Wave Tranduction In Sputtered Lead Zirconate Titanate Thin Films, IEEE 1988 Ultrasonics Synopsium Proceedings, pages 291-295. Unfortunately, the reported literature suggests it has not been possible to achieve PZT thicknesses of greater than about 5 microns with known PZT sputtering methods. In addition, PZT deposition rates using known sputtering methods are unacceptably slow for commercial applications, i.e., no more than 0.5 microns per hour. Due to limitations in thickness of PZT layers and slow deposition rates, known PZT sputtering methods do not present a viable approach to fabricating PZT layers in future-generation ultrasonic transducer arrays. Moreover, it is believed known PZT sputtering techniques have not been used in connection with the manufacture of multilayer ultrasonic transducer arrays.
As the size of discrete transducer elements in transducer arrays decreases, and the number of such elements increases, it becomes increasingly difficult to incorporate the necessary electrical connections and transmit and receive circuitry for each element. One solution to this problem discussed in the literature is to provide a sparsely sampled array in which transducer elements are provided at only some fraction of the available element locations. Further benefits can be obtained by using separate transmit and receive elements that do not share common structure. Unfortunately, prior art designs and fabrication techniques for transducer arrays do not provide viable solutions to the technical hurdles associated with the manufacture of such sparsely sampled arrays.
Generation of transmit and receive signals may be optimized by providing a sparsely sampled array having transmit elements with one structure and receive elements with a different structure. Maximum transmit signal is obtained by matching the impedance of the transducer element to that of the driver using a multilayer PZT ceramic. By contrast, maximum receive signal-to-noise ratio is obtained using a receive single layer PZT transducer element having a relatively high impedance that matches that of the preamp drive and locating the preamp electronics close to the element. Greater open circuit receive signal strength may be obtained with single layer elements.
Unfortunately, known designs and fabrication techniques for multilayer PZT transducer arrays do not lend themselves to the manufacture of sparse arrays of the type described above. Indeed, it is believed sparse arrays having elements with the dimensions and operating characteristics desired in future generation ultrasonic transducer arrays cannot be manufactured using known designs and fabrication techniques.
Another ultrasound application, high-intensity focused ultrasound (HIFU), has significant potential for use in therapeutic ultrasound applications including noninvasive myocardial ablation, drug delivery, drug activation, ultrasound surgery, and hyperthermia cancer therapy. Ideally, HIFU therapies would be performed while simultaneously viewing the area being treated. For example, for therapy, high power sound bursts at one frequency may be required, while for imaging, a different frequency may be desirable to provide images with sufficient resolution. Furthermore, the characteristics of the therapy and imaging ultrasound transducers will be different. A sharp resonance is required for improved efficiency for therapy, while a broad bandwidth is required for effective imaging.
Unfortunately, known ultrasound imaging systems do not typically permit such dual imaging with a single transducer array. Instead, with current systems, the body region to be treated is imaged with a first transducer, and then the HIFU therapy is administered with a second transducer. Introduction of an ultrasound transducer into certain body regions can be a relatively lengthy, e.g., 45 minutes, and risky procedure. Also, appropriate placement of the transducer delivering the HIFU therapy is a challenge given the absence of contemporaneous imaging information. Thus, a need clearly exists for a transducer capable of simultaneously providing imaging information and administering HIFU therapies.
As used herein, the term "1-D array" refers to an array having (N.times.1) discrete transducer elements, the term "2-D array" refers to an array having (N.times.M) discrete transducer elements where N and M are equal or nearly equal in number, and the term "1.5-D array" refers to an array having (N.times.M) discrete transducer elements where N&gt;M, e.g., where N=128 and M=3.